1. Field of the Invention
The invention pertains to the field of microwave attenuators and more particularly to matched microwave variable attenuators in monolithic microwave integrated circuits.
2. Description of the Prior Art
Microwave attenuators of the prior art have one of two classic configurations; the resistive Pi circuit shown in FIG. 1a and the resistive T circuit shown in FIG. 1b. These attenuators utilize either PIN diodes or field effect transistors (FETs) for the resistors R.sub.1 and R.sub.2. PIN diodes and FETs exhibit resistive changes with properly applied DC voltages and thus are useful as variable resistors. In the configurations shown in FIGS. 1a and 1b the resistive values, for all levels of attenuation are chosen to provide an impedance that matches the impedance of a transmission line, or another microwave device, to suppress reflections in the system. To accomplish this, the ratio of R.sub.2 to R.sub.1 must change with attenuation changes, establishing a functional relationship of R.sub.2 /R.sub.1, versus attenuation which is extremely non-linear. This presents a very difficult tracking problem, requiring that the dc characteristics of the PIN diodes or FETs utilized in the attenuators be matched over the entire attenuation range. As a result, the PIN diodes and FETs are generally controlled with separate power supplies. Though control circuitry can be provided to supply the DC voltages to the voltage controlled resistances in their proper functional relationship from a single power supply, such circuitry requires much more real estate than the attenuator it controls and is therefore rejected for most applications.
The problem of providing the proper ratios for R1 and R2 to maintain a constant characteristic impedance for the Pi and T circuits of FIGS. 1a and 1b is exacerbated by the non-uniformity of the PIN diode and the FET characteristics that result with present day manufacturing processes. For example, the equivalent resistance value of a FET is a function of the pinch-off voltage, that voltage which must be exceeded by the gate voltage for current to flow in the FET. Present day manufacturing processes, however, yield FETs with pinch-off voltages that vary substantially. Since the resistance of the FET is a function of the pinch-off voltage, FETs exhibit resistance values having varying functional relationships of the gate voltage. Thus, for each attenuator a process is encountered for selecting three FETs with equal resistance versus gate voltage characteristics, greatly increasing cost of the attenuators.
Further, the resistive Pi and T circuits of FIGS. 1a and 1b cannot simultaneously realize low (off) state insertion loss and a large dynamic attenuation range with the variable resistors presently available. For both circuits a low insertion loss requires a low resistance value for the series elements and a high resistance value for the shunt elements. As attenuation increases from the minimum value the series resistance increases, while the shunt resistance decreases. Since the shunt resistance start at opposite ends of the functionality curve it is extremely difficult to provide the ratio of series resistance to shunt resistance required for the attenuation values desired and simultaneously maintain a constant characteristic impedance for the circuits.
Additionally, at high frequencies, the internal capacitances of the PIN diodes and FETs establish complex characteristics for the Pi and T circuits. To provide real characteristic impedances it is necessary to resonate out these capacitances by shunting inductors across the elements of the Pi and T circuits. These resonant circuits severely limit the operating bandwidth of the attenuator.
It is therefore desirable to supply an attenuator that provides a variable attenuation with the adjustment of a single resistor and exhibits a characteristic impedance that is not a function of this resistive value.